A typical physical vapor deposition (PVD) apparatus includes a processing chamber, a cathode assembly and a substrate support within the chamber, a vacuum system to maintain the pressure in the chamber below 100 mTorr and a gas supply system to introduce a sputtering gas into the chamber. The cathode assembly includes a target, insulators to electrically isolate that target from the chamber wall, a power supply to energize the target, and a magnetron magnet assembly to form a plasma confining magnetic field close to the target surface. When the substrates being processed by the PVD apparatus are silicon wafers for integrated circuit manufacturing, the most commonly encountered PVD apparatus vacuum systems use high-vacuum cryogenic pumps and several pressure gauges.
Because sputtered material is ejected from the target in all directions in the processing chamber of a PVD apparatus, the whole chamber, not just the substrate, is exposed to coating material from the target. Standard practice has been to place physical barriers known as shields inside the chamber so as to prevent unwanted deposition on the chamber walls and on various other components inside the chamber. For example, shields may be used to protect the dielectric insulator that electrically isolates the target from the usually grounded metal chamber walls. Shields used to protect chamber components usually have their surfaces roughened so that material that is deposited onto the shields adheres better to the shields and does not spall as it increases in thickness. If the deposited material does not adhere well to the shields, it can flake off, causing particles that can land on the substrate. In integrated circuit manufacturing, these particles can destroy sensitive devices on the substrate surface. Usually these shields must be changed on a regular preventive maintenance schedule. Otherwise the accumulated deposits will become too thick and stresses will build up that cause the shields to shed particles.
For some sensitive applications, a process chamber must be capable of being evacuated to an ultra-high vacuum (below 10−8 Torr), and the sputtering gas must be purified before it is introduced into the process chamber. The equipment required to achieve these conditions is very expensive. In other semiconductor PVD applications, such equipment is not required. Some less critical PVD applications used for integrated circuit manufacture are extremely sensitive to cost, and call for equipment having a minimum of expensive components. Some of the most expensive components of a PVD chamber are those required to achieve ultra-high vacuum (UHV).
For silicon wafer processing, the process chamber is most commonly pumped by a dedicated high vacuum pump, usually a turbo-molecular or cryopump. However, there are low cost PVD systems such as the Ulvac SRH-820 and the Sputtered Films, Inc. (SFI) ENDEAVOR, that use a single, common, centrally located high vacuum pump to evacuate all PVD process chambers. The lower cost systems that may be used for less critical applications can achieve such pumping with less concern for chamber cross-contamination or interference.
High end, single wafer, PVD tools such as the Tokyo Electron Limited ECLIPSE Series, the Applied Materials ENDURA and the Novellus INOVA, for example, are usually considered too expensive for dedication to low end foundry packaging applications. Many foundries are able to use inexpensive, relatively inferior and lower throughput batch tools for their less critical PVD applications.
Single wafer tools have several advantages over batch machines for silicon wafer processing. Single wafer systems lend themselves readily to statistical process control, since every wafer experiences the same process in the same position in a given process chamber. Also, in the event of wafer breakage, usually all the wafers in a batch will be scrap due to the particles generated when a wafer breaks; in a single wafer system, only the wafer that breaks would be lost. A single wafer tool usually has a higher throughput for larger wafer sizes. As wafer size increases, the number of wafers in a batch must decrease correspondingly. Consequently, there are compelling reasons for current users of inexpensive batch tools to convert to single wafer machines, provided that they are sufficiently inexpensive.
One such inexpensive single wafer tool is the Varian 3180 series tool. This tool is a cassette-to-cassette single wafer PVD tool, where sputtering takes place in a large plenum with four sputtering stations. Features of this machine are described in U.S. Pat. Nos. 4,548,699 and 4,716,815. Each station of this tool is directly opposed to a sputtering target. The wafers rotate sequentially from a first station to a last, and may be subjected to a sputter coating or other process at each of the stations. The plenum is pumped by a single cryopump. One disadvantage to this arrangement has been that all sputtering processes take place in a common ambient, at the same pressure. It is often desirable to sputter different metals in a stack at different pressures, for example, to optimize film stress, but this has not been possible with this kind of common plenum machine. Also, in many of such machines, there has been no easy way to isolate the sputtering ambient of the various chambers. In the event that a metal stack requires reactive sputtering, for example, using a mixture of argon and nitrogen to deposit titanium nitride, the processes in adjacent chambers could be contaminated by nitrogen.
Accordingly, there remains a need for a better way to use a common vacuum pumping system in a multiple-chamber single-wafer tool.